For the aspx code I've added a static blank listitem for each dropdown, updated some of the values of the listitems as there were duplicates (20 was duplicated for listitems with text 21 - 23).
also changed the name of the duration textbox
aspx code:

Seems like you have this question open twice...? Anyway, I made a comment in your other question, http:Q_26888706.html#a35143877 with a server-side solution:

This example uses two text boxes for the times; the CustomValidator's function attempts to parse the text into a DateTime, requiring the text fits the format H:mm - where "H" is a 1 or 2 digit hour between 0 and 23, and "mm" is a 2 digit minute between 00 and 59.

The button's click event handler first checks if the controls on the page passed validation by checking Page.IsValid, and then parses the text of each text box into a DateTime. Subtracting one DateTime object from another gets you a TimeSpan object, and the TotalMinutes property tells you the length of the TimeSpan object in minutes.

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